rmtdos

This is a fork from https://github.com/dennisjenkins75/rmtdos - it allows you to quit the rmtdos-client with the keyboard combination 'CTRL-Q'. I found when ssh'ing from a Windows machine to a Linux machine running rmtdos-client 'ALT-ESC' does not work as 'ALT-ESC' in Windows cycles through open windows in the order they were opened. That's it, the rest of the rmtdos and rmtdos-client programs are the same as the original fork.

Remotely control DOS-based systems from Linux

Tested on FreeDOS-1.3 in QEMU and on a real Pentium-MMX 166 MHz.

"rmtdos" contains a DOS (FreeDOS or MSDOS or equivalent) TSR that allows a Linux system on the same LAN to remotely control the DOS system.

Theory of Operation

There are two main programs, one that runs on DOS (rmtdos.com), and the other that runs on Linux (rmtdos-client). There is also a VGA text mode demo (vga_demo.com) program for testing VGA text video modes and displaying test patterns.

rmtdos.com ("server"):

1. Probes for a local PC/TCP packet driver (or you can specify the exact packet driver IRQ on the command line).
2. Registers to receive raw Ethernet frames with an EtherType of 80ab (also configurable on the command line).
3. Hooks interrupts 08 (BIOS clock) and 2f (DOS multiplex).
4. Goes resident (TSR).
5. Responds to Ethernet packets with the correct EtherType and internal packet signatures.
6. Streams contents of VGA text framebuffer (b800:0000) to any connected clients.
7. Can receive control packets that include keystrokes to be shoved into the BIOS keyboard buffer.
8. Run with -h to see usage and command line options.

rmtdos-client:

1. The client probes the local LAN by broadcasting a special packet for the same EtherType.
2. Presents a cheesy ncurses UI that lists discovered servers, and allows the user to select one (just press the key for the server ID, 0-9).
3. Periodically sends a refresher packet to the server the user wants to "connect" to.
4. Receives VGA text memory dumps, and renders them via ncurses.
5. Run with -h to see usage and command line options.
6. Run with -u to uninstall/unload the resident program.

Usage

On the DOS system, install a PC/TCP packet driver, then run rmtdos.com. It should find the packet driver, initialize, "go resident" and return control to DOS.

On Linux, run the client. You might need to override the default ethernet device name. Sadly, you'll need to run it as root since it needs to create a raw socket. sudo ./rmtdos-client -i br0.

The displayed columns are:

1. Ordinal id for the DOS system (0 .. n-1)
2. MAC address
3. VGA mode (text modes are 0,1,2,3).
4. Width and Height of the screen.
5. Time since last packet received from the DOS system.

Then select the client that you want to connect.

Building

1. Install "dev86", which provides a 16-bit x86 compiler, assembler, and linker.
2. make
3. sudo make setcap (optional, see https://stackoverflow.com/a/46466642).
4. Copy out/rmtdos.com and out/vga_demo.com (optional) to a DOS system.

There are some non-traditional makefile targets that I find handy during development:

1. make format - Reformats the C code w/ clang-format.
2. make setcap - Runs the setcap command on the client, to enable a non-root user to run it without requiring sudo.
3. make typos - Runs typos on the source files, finding spelling mistakes.
4. make bcclint - Horrible hack that attempts to compile the DOS binaries with actual GCC (it will fail to produce a binary). This is useful because dev86's compiler, bcc, is a traditional K&R compiler, which will unhelpfully assume function prototypes if you forget to include the proper header. Modern gcc -Wall can be much more strict and issue all kinds of warnings for bad coding practices.

Future Plans

1. Add mouse support. DOS has int 33h with lots of support for setting the mouse's cursor position.

2. Possibly add remote file transfer capacilities (kind of like TFTP over raw Ethernet, maybe with a Linux-side FUSE driver).

FAQ

Q: Why was this project developed using "dev86", which has poor support for modern C?

A: I prefer to do development from my Linux system, and treat DOS as an embedded system that I can push binaries to. So I needed a C compiler that can run on Linux and produce DOS real-mode programs. GCC cannot do this. I attempted and failed to get OpenWatcom 1.9 and 2.0 to compile cleanly on Gentoo Linux, so I used "dev86", which exists for both Linux and DOS.

Q: Why did you use raw Ethernet packets and not TCP/IP or UDP?

A: Memory overhead. A primary design criteria was to keep the memory usage of the resident rmtdos.com program as small as possible. Basically, there were four options for implementing full TCP/IP support, and each of them would have added serious developmental delays and RAM costs:

1. mTCP - Would not work in TSR mode without significant code changes. mTCP assumes that it can issue DOS file IO requests "whenever", and that is not compatible with a TSR that does not want to bother switching out the DOS PSPs and such.

2. Wattcp - Unmaintained, didn't even go there.

3. TSR TCP/IP implementation - It wants 73K of RAM at runtime.

4. Roll my own. Um, no thanks.

A: Compatibility with other mTCP programs. mTCP tells the packet driver that it wants to register for ALL EtherTypes, but it only really uses "ARP" (0x0806) and "IPV4" (0x0800). If rmtdos.com registers for the same, then the packet driver won't let any mTCP program successfully run. Since rmtdos.com uses its own EtherType, both stacks can co-exist on the same packet driver and not require a packet multiplexor.

Q: Can I build it on DOS?

A: rmtdos.com and vga_demo.com might build there, but I've not bothered trying. Currently, all 3 programs are built on Linux. The DOS programs are built using "dev86". FreeDOS ships with "dev86" (if it is not already installed, you can install it with the FreeDOS utility fdimples.exe).

Q: How do I exit the client?

A: Press ALT-ESCAPE. The client sets ncurses to "raw" mode, so that the typical termios control characters (ex: CTRL-C) do NOT generate signals, but instead are presented to the client, so that the client can send them to the DOS end.

Q: How can I view the raw Ethernet traffic (tcpdump filter expression)?

A: tcpdump 'ether proto 0x80ab'

A: (wireshark): "eth.type == 0x80ab"

Q: My DOS system does not have a NIC, can I use a serial port?

A: In theory, yes; but I've not tested this. Connect the DOS system to your Linux system via "null-modem" cable. On the DOS side, run the "CSLIP" packet driver. On the Linux end, configure "cslip" on the serial TTY device. This guide contains a step-by-step walkthrough:

• https://mcmackins.org/stories/dos-slip.html

Q: rmtdos.com is optimized for minimal memory usage. How did you audit that?

A: Well, its a goal. I'm sure that there is always room for improvement. I've been auditing the memory usage by examining the linker map via:

• make && sort -k3 out/rmtdos.map

And by using debug.com to peek at the TSR's private ISR (interrupt service routine) stack usage (to see if it is safe to shrink the ISR stacks).

1. Build the software, examine the linker map:

                   int08   _int08_stack_bottom  3  00003020  R
                   int08      _int08_stack_top  3  00003220  R
                   int2f   _int2f_stack_bottom  3  00003224  R
                   int2f      _int2f_stack_top  3  000032a4  R
                 pktrecv   pktdrv_stack_bottom  3  000032a8  r
                 pktrecv      pktdrv_stack_top  3  000033a8  r

2. Then load rmtdos.com in DOS. Observe the printed PSP value.

3. Connect with the client from Linux. This will cause rmtdos to start using its ISR stacks.

4. Run debug.com, and examine the RAM where the stacks are (example is for pktdrv_stack_bottom:

C:\TEMP>rmtdos
Packet Driver Initialized.  irq:0x60, NE2100, 82:00:00:00:00:09, et:80ab
Going resident.  PSP:073b, Last Addr: 4c50

C:\TEMP>debug
-d 073b:32a8
073B:32A0                         -00 00 00 00 00 00 00 00         ........
073B:32B0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:32C0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:32D0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:32E0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:32F0  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:3300  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:3310  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:3320  00 00 00 00 00 00 00 00-                        ........
-d
073B:3320                         -00 00 00 00 00 00 00 00         ........
073B:3330  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:3340  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:3350  00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
073B:3360  00 00 00 00 00 00 4E 3C-4E 3C 54 3C 90 3C 78 33 ......N<N<T<.<x3
073B:3370  57 1E 54 3C 54 3C 90 3C-82 33 FD 2A 82 33 3B 07 W.T<T<.<.3.*.3;.
073B:3380  82 33 01 00 40 01 3C 00-00 00 54 3C 90 3C 82 33 .3..@.<...T<.<.3
073B:3390  82 33 00 00 3B 07 3B 07-3B 07 3B 07 3A 0A 00 00 .3..;.;.;.;.:...
073B:33A0  C6 02 E6 00 64 0D 00 00-                        ....d...